"Inspired by true events" — a phrase that implies the greatest possible distance between something that actually happened and what's about to happen on screen — Snitch tries to be two movies at once.

One is an earnest social drama about the cruel, arbitrary nature of mandatory minimum sentences for first-time drug offenders. The other is an action movie starring Dwayne Johnson, the sometime wrestler popularly known under his nom de headlock, The Rock. Rather than attempting to reconcile these two very different agendas, co-writer and director Ric Roman Waugh opts instead for a cake-and-eat-it approach that compromises Snitch from both ends.

Whatever was relatable and down-to-earth about a father doing everything possible to save his son from injustice is obliterated by a succession of big shoot-'em-up set pieces that no normal dad could expect to survive. And whatever lizard-brain fun might have been had in watching Johnson do battle against a drug cartel is weakened by the occasional hard tug at the social conscience. The film winds up divided against itself.

As John Matthews, the owner and operator of a small construction business, Johnson plays a self-starter whose belief in toughness and discipline doesn't resonate with his 18-year-old son Jason (Rafi Gavron). When Jason gets coerced into accepting a drug shipment that's being monitored by the feds, the quantity of drugs in the package requires a judge to give him a 10-year sentence, despite his otherwise clean record. His only chance to reduce the sentence is to set up a friend or lead the authorities to a bigger fish, which he is, respectively, unwilling and unable to do.

"Into this situation charges John, eager to redeem himself to the son he's been neglecting in favor of a new wife and family. John manages to convince a federal prosecutor (Susan Sarandon) to allow him to frame a drug dealer in exchange for a lighter sentence."

Uncertain at first — he actually starts by looking up "drug cartel" on Wikipedia — John presses one of his ex-con employees (Jon Bernthal) to introduce him to local slingers and earn their trust as a courier. But as he goes further up the chain, John's life as an informant grows more perilous for him and his family, and it's not long before he's in way over his head.

Waugh doesn't do enough to emphasize John's vulnerability in this situation, though his star's musclehead presence may have something to do with it. John may be a tough guy — and the film is relatively subtle about how that braggadocio works to alienate his family — but ordinary people generally subjected to a fusillade of machine-gun bullets or car chases out of The Fast and the Furious. A father summoning the courage to save his son is one thing, but feats of superhuman strength and agility take him out of the everyday realm. Dwayne Johnson can be a fine, effortlessly charismatic actor, but for much of Snitch, he's The Rock.

Still, there's much to savor on the sidelines, particularly in supporting performances that suggest a richer, pulpier thriller than the movie ultimately turns out to be. Buried behind a scraggly goat beard, Barry Pepper brings soul to an undercover drug operative who shrewdly attempts to satisfy the federal prosecutor while protecting his informant from a near-suicidal mission. And Michael K. Williams, now and forever Omar from HBO's The Wire, brings his trademark swagger to a midlevel dealer who's angling to get ahead.

But such grace notes are overwhelmed by the noise Waugh traffics in on Johnson's behalf. Snitch wants to shine a light on heavy-handed sentencing laws and the absurd lengths necessary to reduce time, but it's too big a movie for that story. Sober messages on social justice may come in many forms, but the dumb spectacle of a runaway semitruck swatting cars off the expressway isn't one of them.